Numerous forms of plastic tie strip are described in the prior art with many varieties being manufactured by injection-moulding from suitable polymer materials. Cable ties (e.g. U.S. Pat. No. 3,186,047, Schwester et al) are a classic example of the technology, as generally shown in FIG. 1. They have a head and a long flat tail, the head having a corresponding slit through which the tail can pass, with a ratchet arrangement to hold the tie in place. They are conventionally produced by standard injection-moulding processes, using either cold or hot runner systems, the latter commonly being the most suitable method for mass production.
The moulding process for these forms of tie strip is normally to fill the mould cavity through a single small orifice cut into the mould known as a gate 1, the gate usually being sited at one end of the tie (FIG. 2).
This allows the polymer melt to flow to the other end of the part, thereby filling the cavity without voids. By locating the gate at the end (a functionally uncritical zone), any residual material or gate vestige at the point of entry will not impede the threading of the tie when in use.
An alternative location for the gate—especially on very long ties—is roughly halfway along the length, as shown at 2. The polymer melt then bifurcates to flow to either end, thus halving the distance it must travel and thereby reducing the injection pressure requirements. Normally such a gate would be located in a place that minimises any gate vestige. Often this is on the narrow side of the tie, as shown by reference 3 in FIG. 3, with the gate kept as small as practical. Tab gates with minimal land 4 may be employed in this respect. Alternatively, the mould may be gated on the top surface FIG. 4, and the gate may be sunk into the tie 4 to keep any gate vestige below the working surface. Pin gates are the usual method employed here.
A more versatile type of tie strip is described in prior art such as U.S. Pat. No. 3,438,095 (Evans), U.S. Pat. No. 5,799,376 (Harsley) and WO 2004/108550 (Harsley), the latter being illustrated in FIG. 5. The strip has a relatively short threading end or tongue 9, and a long tail of constant width formed of a multitude of identical cells 101, through any one of which the tongue can pass, followed by some cells; the cells each have retaining arms or hooks 103 to keep the tie closed.
These forms of tie are also often produced by injection moulding, but their complex geometry results in very long and winding flow paths for the polymer melt. This leads to manufacturing difficulties because much higher injection pressures are required. Although a central gate will somewhat reduce the required injection pressure (halving it, to a first approximation), this is still not sufficient for these types of tie strip. Because injection-moulding machines have limiting injection pressures and clamping forces, the total number of cavities that can be filled decreases as the pressure required to fill them increases. That is, high pressure requirements will limit the number of parts that can be made in each moulding cycle. Hence, for any given machine, improved productivity can only be attained by lowering injection pressures, and this calls for multiple gates 5, as shown in FIG. 5.
Apart from the associated cost of a complex multiple-drop hot-runner system, the practical problem with additional gates is that gate vestiges 6 are left along the working length of the strip, as shown in FIG. 6, and these can lead to damage of the thin walls 102 of the tie during fitting or tying—see FIG. 7. This occurs as the strips are pulled through one of the cells to form a loop. Any protruding gate vestiges can cut the insides of the walls, 7, and this often weakens or breaks the cell 8.
Countersinking top gates to avoid gate vestiges is only practical on sufficiently thick ties, and, since such ties have larger wall sections, they have improved flow characteristics and do not generally need so many gates anyway. They are also more rugged and less prone to damage in the first place.
It is therefore with the thinner tie strips that the problem lies; they require more gates to fill and are more susceptible to damage from the resulting vestigial protrusions.